This invention relates to a heat transfer sheet, and more particularly to a heat transfer sheet which is particularly suitable for obtaining an image on a heat transferable sheet by carrying out heating printing corresponding to image information by a thermal head or laser.
Further, the present invention relates to a process for producing a polyvinyl acetacetal resin, particularly to a process for producing a polyvinyl acetacetal resin of high acetalation degree and little irregularity of particle size.
For obtaining an image corresponding to an information image by the use of a heating printing means such as thermal head or laser, heat-sensitive color forming has been primarily used in the prior art. In such heat-sensitive color forming paper, a leuco dye which is colorless or pale yellow at room temperature provided on a substrate paper and a developer are brought into contact by heating to give a color formed image. As such developers, phenolic compounds, zinc salicylate derivative, rosin, etc. have been generally employed. However, a heat-sensitive color forming paper has a serious defect of color extinction when the color formed image is stored for a long term, and color printing is limited to two colors and cannot give a color image having continuous gradation.
On the other hand, a heat-sensitive transfer paper having a heat-fusible wax layer comprising a pigment dispersed therein and provided on a substrate paper has begun to be used in recent years. When the heat-sensitive transfer paper is superposed on a heat transferable paper and heating printing is carried out on the back surface of the heat-sensitive transfer paper, the wax layer containing the pigment migrates onto the transferable paper to produce an image thereon. According to such a printing method, by performing printing a plural number of times by the use of a heat-sensitive transfer paper containing pigments of three primary colors, a multi-color image can be obtained, but it has been impossible to obtain a photograph-like image having essentially continuous gradation.
In recent years, there has been an increasing demand for a technique to obtain photograph-like images directly from electrical signals, and various attempts have been made. One of such attempts is a method in which an image is formed on CRT, and this is photographed with a silver salt film. However, when the silver salt film is an instant film, there is a drawback in that running cost becomes high, while, when the silver salt film is a 35 mm film, there is the drawback of lack of instancy because a developing processing is required after photographing. As still another method, the impact ribbon method or the ink jet method has been also proposed. However, the former involves the drawback of bad image quality, and the latter a drawback in that image cannot be obtained as simply as photography because image processing is required.
For solving such problems, there has been proposed a method in which a heat transfer sheet having a sublimatable disperse dye layer having the property of migration by heating provided thereon is used in combination with a heat transferable sheet, and the sublimatable dye is caused to migrate under control onto the heat-transferable sheet, thereby obtaining an image having gradation like a photograph (Journal of Image Electronic Society, Vol. 12, No. 1, 1983). According to this method, an image with continuous gradation can be obtained by simple processing from television signals, and yet the device used thereby is not complicated, whereby this method is now attracting attention.
As one of the prior art techniques approximating this method, the dry transfer printing method of polyester fibers may be mentioned. This method is a method for obtaining an image, which comprises dispersing or dissolving a dye such as a sublimatable disperse dye, etc., in a synthetic resin solution to prepare a coating material, applying the coating material in a pattern on a thin paper or like material, drying to form a heat transfer sheet, superposing the heat transfer sheet onto a polyester fiber which is the heat transferable sheet, and heating the composite under adhesion, thereby applying the disperse dye onto the polyester fiber. However, even when the heat transfer sheet used in the prior art for the dry transfer printing method of polyester fibers is used as it is, and heating printing is carried out by a thermal head or the like, it is difficult to obtain a color formed image with high density. The reason for this may be the fact that the heat-sensitivity of the heat transfer sheet is not high, and the dyeing ability of the heat transferable sheet is low.
Of these drawbacks, those caused by the heat transferable sheet were found to be solved by a heat transferable sheet having a heat transferable layer comprising the island portions independent of each other comprising a synthetic resin having a glass transition temperature of -100.degree. C. to 20.degree. C. and a polar group and a sea portion comprising a synthetic resin having a glass transition temperature of 40.degree. C. or higher formed as an island-sea configuration (Japanese Patent Application No. 135627/1983), but those caused by the heat transfer sheet have not yet been solved. This is because, in the method of printing onto fibers, etc., migration and transfer of the dye are accomplished by heating, for example, at 200.degree. C. for about 1 minute, while heating with a thermal head is as short as several msec. at about 400.degree. C.
We have carried out various studies to obtain a heat transfer sheet which can be suitably used in combination with a heat transferable sheet, particularly a heat transferable sheet of Japanese Patent Application No. 135627/1983 as mentioned above, in order to obtain an image of color photographic tone by heating printing with a thermal head, etc., and consequently found the following facts.
In the heat transfer sheet generally used in the prior art, the disperse dye exists in a state wherein it is dispered as particles in a binder, and for sublimating the dye molecules under such a state by heating, heat energy breaking the interactions within the crystals and further surpassing the interaction with the binder must be imparted to the dye molecules to accomplish sublimation and dyeing thereof onto the heat transferable sheet, whereby high energy is required. Also, in the case where, in order to obtain a colored image with high density, the dye is contained in the binder resin at a high relative ratio, an image with a somewhat high density can be obtained. However, because of the weakened bonding force in the heat transfer layer in the heat transfer sheet, when it is peeled off after printing with a thermal head with a heat transferable sheet superposed thereon, a phenomenon wherein the transfer layer is taken over together with the resin onto the heat transferable sheet is liable to occur. Further, since the dyes are of high cost, it is also disadvantageous to incorporate more dyes than necessary from the standpoint of such intended purposes as OA instruments or home uses.
On the other hand, if it is possible to maintain dyes in a binder in molecular dispersed form instead of particulate form, improvement of heat sensitivity corresponding to the absence of the interaction within the crystals as in the case of particulate dispersion may be expected. However, even when such a state is merely attained within a binder, a practically useful transfer paper cannot be obtained. More specifically, thermally sublimatable dye molecules have relatively smaller molecular weights of about 150 to 550 and are mobile in the binder. Accordingly, for example, when a binder with a low glass transition temperature (Tg) is employed, there occurs the phenomenon wherein agglomeration occurs with elapse of time to cause precipitation, resulting ultimately in the state of the dyes being dispersed in particulate form as described above, or due to bleeding onto the surface of the heat transfer layer, the dyes adhere also around the heating portion by the pressure between the thermal head and the platen (pressurizing plate) during recording, whereby ground staining is generated to cause serious deterioration of the image quality.
Also, even if the glass transition temperature (Tg) of the binder is high, the dye molecules cannot be retained unless the molecular weight of the binder is large to some extent. Further, even when the dye is dissolved in molecular state in a binder with a high glass transition temperature (Tg) and somewhat large molecular weight, affinity between the dye molecules and the binder is required in order to attain a state which is stable with time.
In view of such points, various heat transfer sheets intended to improve image quality have been proposed. For example, as described in Japanese Laid-Open Patent Publication No. 101087/1985, it has been known to obtain improvement of printing quality and stability with time by the use of a specific polyvinyl butyral resin as the binder component in the ink composition. However, these heat transfer sheets of the prior art are not necessarily amply satisfactory with respect to storability.
Generally speaking, as the conditions demanded for heat transfer sheet, storability of the heat transfer sheet itself is important along with various characteristics participating in image quality such as printing sensitivity and resolution. However, printing sensitivity and storability greatly tend to cancel each other, and it is difficult to improve both of these characteristics.